Microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS), used for producing sensors or actuators, normally comprise a fixed part and at least one part suspended with respect to the fixed part. The suspended part is able to move and/or deform under the effect of an external force that may be of mechanical, electrical or magnetic origin.
An article by P. Robert et al. (“M&NEMS: A new approach for ultra-low cost 3D inertial sensor,” IEEE Sensors Conference 2009, 25-28 Oct. 2009), describes a structure comprising MEMS and NEMS devices, forming an accelerometer. The structure comprises an active part formed by two distinct thicknesses. The NEMS device, which forms a strain gauge, uses a first thickness, and the MEMS device, which forms a seismic mass, uses the first and second thicknesses (or in other words, uses the whole of the active part). Such an active part can be produced from an SOI substrate that defines a first layer having the first thickness. An epitaxial growth step is next implemented in order to produce a second layer having the second thickness. This second thickness is normally thicker than the first thickness. The second thickness is typically around a few tens of microns, compared with at least one micron for the first thickness. The epitaxial growth step for these ranges of thicknesses is therefore lengthy and expensive. Moreover, the epitaxially grown layer contains polycrystalline regions because of the presence of a discontinuous layer of oxide on the first layer (SOI) for defining the devices. These polycrystalline regions may give rise to defects in the structure, impacting functioning of the end devices.
An alternative method for manufacturing such a structure comprising MEMS and NEMS devices is disclosed in European Patent No. EP 2599746. The method comprises the production, on a first monocrystalline semiconductor substrate, of a locally porous region or of a region locally implemented with a plurality of pillars. Next, an epitaxy on this substrate makes it possible to form the first layer having the first thickness. This first layer is then worked to define the NEMS device and to leave clear a membrane, by etching the locally porous region or the region with pillars, used as a local sacrificial layer. A deposition of oxide is next carried out in order to reblock the openings (under the membrane) and to create a sacrificial layer over the entire surface of the first substrate, above the first layer, and, therefore, in particular on the NEMS membrane. The sacrificial layer of oxide is assembled on a supporting substrate, and then the first substrate is thinned by its rear face in order to form the active part. The thickness of this active part is the sum of the first and second thicknesses. The thinned face is worked, in order to define the NEMS device and to remove the second layer having the second thickness above the NEMS device, stopping on the oxide layer that was used previously to reblock the openings. Finally, the membranes at the NEMS and MEMS devices are released by local removal of the sacrificial layer of buried oxide.
This method requires several steps of lithography, etching and deposition in order successively to define, work, and release membranes and then encapsulate them in a sacrificial layer, before bonding on the support substrate, which gives rise to high manufacturing costs. Moreover, the predefinition of the NEMS device on the first substrate, before bonding on the support substrate, may give rise to loss of efficiency. First, because the bonding step is very sensitive to any residue of topology, roughness or particles, and second, because any defect in alignment between the NEMS (buried) and MEMS devices during the working of the thinned rear face of the first substrate may impact functioning of the end device.